New dual emission fluorescent sensor for pH and Pb(II) based on bis(napfthalimide) derivative

This paper describes a novel dual emission bis-1,8-naphthalimide sensor for selective determination of pH and Pb2+ ions. The influence of the variability in the backbone that links the two fluorophores (naphthalimides) as a function of pH and metal ions was studied by UV–visible and fluorescence spectroscopy. Compounds 1(a–d) with different length alkyl linkers (CH2)n (n=1, 2, 4 and 6) showed no excimer formation in aqueous solution. Fluorescence emission of these derivatives varied in a narrow range of pH (5–8) and was only slightly influenced by the addition of metal ions in CH3CN solutions. However, derivative 1e with amino-containing spacer (CH2–NH–CH2) showed excimer emission in aqueous solution, a wide response to pH (2.5–9.5) and fluorescence enhancement with selective behavior towards metal ions. The pH sensor based in derivative 1e has a sufficient selectivity for practical pH monitoring in the presence of Li+, Na+, K+, Cs+, Ca2+, Mg2+, Ba2+, Cu2+, Pb2+, Ni2+, Zn2+ and Cd2+. The coordination chemistry of these complexes was studied by UV–Vis, fluorescence and 1H NMR. This chemosensor displayed high selectivity fluorescence enhancement toward Pb2+ ions in the presence of the metals ions mentioned in CH3CN solutions. Competitive assays show that a 1-fold of metal cations in each case, compared with Pb2+ ions, results in less than ±5% fluorescence intensity changes. Linear calibration up to 1×10−5 M for Pb(II) ions (R=0.9968) was obtained and detection limit resulted of 5.0×10−8 M.